Mechanical ventilation may be performed on a patient during the operation of a medical instrument such as a ventilator or an anesthetic machine. For a patient under treatment, especially an infant patient and a seriously ill patient, it is important to control a ventilation volume and adjust an oxygen concentration. The precision and the grade of the ventilator depend on the precision, sensitivity and continuous adjustment of an oxygen concentration with respect to both a large tidal volume of the adult and a small tidal volume of the infant.
The tidal volume generally refers to the amount of air inhaled or expired during each human breath in a resting state (i.e. a state without stimulation), and depends on age, gender, volume surface, breathing habit and metabolism of the body. The preset tidal volume generally refers to the amount of the inhaled air. The preset tidal volume, which is not constant, is adjustable according to the blood gas analysis of the patient, and normally is: 8-10 ml/kg for an adult and 10-15 ml/kg for an infant.
Presently, the conventional oxygen mixing valve is generally designed so that the mixed proportion of the air and the oxygen is controlled by two flow proportional control valves or end-face seal valves. In the first design of such two designs, two flow proportional control valves are adopted, thus the product manufacturing cost is increased, and it is very difficult for the first design to precisely control the precision of the small tidal volume of the infant. The second one of such two designs adopts two end-face seal valves which are demanding for the precision of manufacturing and assembling of parts, thus the manufacture becomes difficult and the sealing is not good, resulting in an unstable mixed proportional relation of the air and the oxygen.
As shown in FIG. 1, Chinese patent No. CN101766861A discloses an air mixing module and a ventilator and an anesthetic machine with the same, in which the air mixing module includes a housing 5a which has a mixing cavity 52a and a first cavity 51a and a second cavity 53a arranged at both sides of the mixing cavity 52a, the first cavity 51a and the second cavity 53a are in communication with the mixing cavity 52a through a first gas outlet 512 and a second gas outlet 532, respectively, a core shaft 2a passes through the first cavity 51a, the mixing cavity 52a and the second cavity 53a in sequence along the axial direction and has a shoulder 22a received in the mixing cavity 52a; furthermore, the air mixing module further includes a step motor 9a used to drive the core shaft 2a to move in the axial direction, thereby simultaneously changing the opening degrees of the first gas outlet 512 and the second gas outlet 532 through the axial movement of the shoulder 22a. 
The above solution adopts the step motor 9a for controlling the core shaft 2a to both rotate and displace, through transmission in the form of trapezoidal thread transmission and a gas mixing mechanism kinematic pair in the form of a fixing nut 4a. The core shaft 2a of the gas mixing mechanism is connected with the shaft of the step motor 9a through an elastic coupling 1a, of which the torsional rigidity is sufficient for converting a helical motion into a linear motion, the position of the shaft of the motor can be determined by an opposite-type photoelectric switch 3a, a limit thread 6a and a photoelectric stop plate 7a, etc., and the step motor 9a and the core shaft 2a are positioned by a one-piece bracket 8a, two coaxial holes of which ensure the coaxiality of the motor shaft and the core shaft 2a. An air-oxygen mixing mechanism, which is arranged following an air-inlet module, forms a part of an interior air path of the ventilator before an air intake tube of the ventilator; the air and the oxygen from external gas sources pass through the air-oxygen mixing mechanism, the position of the core shaft 2a within the cavity is precisely controlled by the step motor 9a and the coupling to simultaneously adjust the flows of air and oxygen to achieve the oxygen concentration needed by the patent, and then the tidal of a volume needed by the patent is outputted by a flow proportional valve to an air-intake pipeline.
The disadvantages of the above solution lie in that: firstly, the seal effect is not good enough since the air outlet and oxygen outlet are sealed by only the end-face seal through the shoulder 22a of the core shaft 2a; secondly, such end-face seal is demanding for the flatness of the contact surfaces, thus the manufacture becomes difficult; and thirdly, it is difficult to align the center of the core shaft 2a with that of the motor shaft, such that the core shaft 2a is easily not concentric with the motor shaft, causing the poor seal between the shoulder 22a and the first gas outlet 512 or the second gas outlet 532, as a result, gas leakage is caused and the gas proportions are out of the required ranges.